A protective relay device is installed for the purpose of maintaining stable operation of an electric power system, when a fault which has occurred in a busbar, transformer, power transmission line, or other electric power system equipment is detected, by detaching the fault equipment from the robust electric power system.
In a important electric power system equipment, often a current differential protective relay device with excellent protection performance is applied as a protective relay device.
A current differential protective relay device takes as inputs the currents flowing at both ends of an equipment to be protected and performs a differential operation to detect interior and exterior faults, and so when applied to protection of power transmission lines, synchronizes with both terminals of the power transmission line and transmits and receives an instantaneous current value data acquired by its own terminal and the other terminal and takes as inputs the instantaneous current value data of its own terminal and the transmitted instantaneous current value data of the other terminal, performs a differential operation, and detects operation. When a current differential protective relay device for a power transmission line is a digital device, a PCM (Pulse Code Modulation) transmission method is adopted as a method of transmission of an instantaneous current value data (see for example Non-patent document 1).
This PCM transmission method is a method in which an instantaneous current value data signal is sampled at a sampling frequency based on signals within the protective relay device, and after conversion into digital signals, the data signal is encoded and transmitted.
Because a system current is sampled at intervals of an electrical angle of 30°, the sampling frequency in conventional PCM transmission methods is set to 600 Hz in the case of a 50 Hz system, and to 720 Hz in the case of a 60 Hz system.
Further, in current differential protective relay devices, as means of synchronizing the data sampling times at individual terminals, means of measuring and correcting time differences of sampling timing between opposing devices is employed, so that fixed conditions on the transmitting side are required. Further, 54 kbits/second is stipulated as the transmission speed, and the amount of data which can be transmitted per sampling is stipulated to be 90 bits in the case of 50 Hz and 75 bits for a 60 Hz system.
FIG. 11 shows an example of a transmission format to transmit an instantaneous current value data signal, in a conventional PCM transmission method used in a 50 Hz system.
FIG. 11 shows an example of transmitting one sampling data of three-phase currents, using a frame with 90 bit length. In the frame, “1” bits inserted at 12-bit intervals are called fixed bits, and are used for frame identification.
Recently, sampling has come to be performed with the electrical angle changed from 30° to 3.75° and at the sampling frequency of 4800 Hz in the case of a 50 Hz system and 5760 Hz in the case of a 60 Hz system. Further, conversion into digital signals, conventionally has been performed using a resolution of 12 bits, but recently, conversion has come to be performed using a resolution of 16 bits. However, because the amount of data which can be transmitted is unchanged, it is necessary to take the trouble to convert 16-bit data into 12 bits and perform transmission (see for example Patent document 1).    Non-patent document 1: Toshiba Review, Vol. 41 No. 11, November 1986, pp. 942-945    Patent document 1: Japanese Patent Application Laid-open No. 2000-152486
As explained above, in conventional technology, even when sampling is performed with high precision and digitally converted data is acquired by a protective relay device, due to constraints on the communication system resulting from the sampling synchronization means, there has been the problem that a high-performance protective relay device capable of executing complex processing cannot be constructed.